Presently, microcomputer systems own the capabilities of rapid computing and data storage. In current mechanical and electrical systems, controllers composed of microcomputers have long replaced the purely mechanical or the electrical and mechanical control mechanisms in the past time. The signals in microcomputers are digital, namely, the logic “0” and “1”. The logic “0” represents the low voltage level, which is usually 0 volt in microcomputer systems; the logic “1” represents the high voltage, which is usually 5 volts in microcomputer systems. Nonetheless, physical phenomena in the natural world usually exhibit continuous input signals when they are expressed in quantities. Thereby, it is required to convert the signals for transferring variations of physical quantities in the outside world to microcomputers and computing or for driving devices pursuant to the commands of microcomputers.
The continuous signals of generally measured voltage or current are called input signals. A device converting input signals to digital signals is named an analog-to-digital converter (ADC). Various circuit structures can achieve the work of analog-to-digital conversion, including single-slope integrating ADC and double-slope integrating ADC.
Every ADC has an integrating circuit. As shown in FIG. 1, the input signal Iin is integrated by the integrating circuit comprising a capacitor C and an operational amplifier 12. The integration signal is produced at the output D of the operational amplifier 12. As shown in FIG. 2, the integration signal is a triangular-wave signal. A comparator 14 compares the integration signal to a reference signal Vref for producing a comparison signal. A counter 16 is coupled to the output of the comparator 14 for counting the comparison signal and thus counting a triangular-wave signal. Thereby, a digital signal can be produced. The comparison signal is further used as a reset signal RST for resetting the integrating circuit and producing a next integration signal, namely a next triangular-wave signal.
According to the above description, every time the comparator 14 compares the integration signal the reference signal and produces a comparison signal, the integrating circuit has to be reset for producing a next integration signal. Thereby, for a 12-bit ADC, the counter 16 has to count for 4096 times, and hence the integrating circuit has to be reset for 4096 times. The integrating circuit according to the prior art produces the integral nonlinearity (INL) error, which is accumulative. Therefore, the INL error increases according to the number of times of resetting the integrating circuit, deteriorating the accuracy of the ADC. Consequently, how to reduce the INL effect has become a major issue of modern ADCs.
Accordingly, the present invention provides an analog-to-digital converting circuit, which can reduce the number of times of resetting an integrating circuit, and thus reducing the problem of INL effect as described above.